Methyl cellulose is prepared by etherifying alkali cellulose with methyl chloride at temperatures of about 50 to 90° C. as described in Examples of JP-B 7-119241. After the etherifying reaction, the reaction mixture is introduced into an agitator vessel containing water at about 95° C. where the salt formed during the reaction is dissolved in water. The suspension exiting the agitator vessel is then subjected to a separation operation in order to obtain the desired pure cellulose ether.
In the separation operation, the use of rotary pressure filters is considered. However, the customary textile filter coverings for rotary pressure filters suffer from the problem that products having a high dissolution temperature penetrate and dwell in the filter so that the filter is clogged and becomes inoperative within a short filtering time. The clogging may be cleared by washing with high-pressure steam or hot water, the customary mesh size filter can be stretched in mesh size or even ruptured. The problem is overcome in U.S. Pat. No. 4,954,268 or Japanese Patent No. 2,895,084 by using a filter of multi-layer sintered metal structure having an increased strength. The multi-layer structure leaves a problem that once the filter is clogged with the product, it is difficult to remove the clogging product by washing.